The present invention relates to sheet material for thermoforming containers in general, and more particularly to sheet materials which are suitable for recycling.
Polyethylene terephthalate (PET) is a plastic resin widely used in consumer products and containers, such as in many disposable beverage containers. PET plastic offers the desirable property of being readily recyclable. Systems are in place in many regions of the world to collect, aggregate, and transport the used PET material objects and to submit them to processors for conversion into flakes for reuse in other applications. This recycled PET (RPET), can then be extruded into sheet form or otherwise formed into other objects, reducing the quantities of plastic waste committed to incineration or landfills. PET is well suited to being prepared in a transparent or amorphous form, and may readily be molded into a desired shape through conventional molding techniques such as thermoforming.
Prior to the development of widespread and accessible plastic recycling systems, other plastic materials were frequently used in consumer products and containers, among them Polyvinyl chloride (PVC) plastic. PVC plastic is also readily thermoformed into desired shapes, and is also adaptable to forming sealed containers by using a variety of heat sealing techniques to cause thin sheets of like PVC plastic to be welded together to thereby form a tamper-resistant seal. Yet recycling techniques for PVC plastic are not as advanced as those for PET plastic, and can be costly. As a result, in many applications there is a desire to substitute PET plastic materials where once PVC plastic had been used. There are many advantages to using a recognized environmentally friendly material such as PET.
Many retail containers are fabricated of transparent sheet material which permits the merchandised contents to be directly viewable by the consumer. To prevent pilferage, products, especially small and valuable items, may be enclosed in larger plastic containers which have sealed margins which may only be opened by a sharp implement or shears. This sealing is usually brought about at the point of filling the container with product by subjecting the overlapping flanges of a clam-shell or two-part container to any of a number of heat-sealing processes. These processes include radio frequency heat sealing, ultrasonic heat sealing, bar heat sealing, and UV sealing. The equipment to carry out these processes is well known in the art, and has been employed in many installations for years, and has been acquired often at great cost. Some of these processes, while providing exemplary service in joining PVC materials, function very poorly with conventional PET materials.
There are two types of joints or connections between plastic elements which in common language are referred to as heat seals, yet which are of a radically different nature. The first is of a primarily temporary nature, and may be referred to as a peelable seal. In a peelable seal two plastic elements are joined by being heated, but the bond between the two elements is weak. As a result, when the bond is subjected to pulling forces the bond between the two elements will fail well before the failure of the underlying plastic. The second type of bond, also sometimes referred to as a heat seal, is of a more permanent nature, and will be referred to herein as a “weld”. In a welded heat seal, the two plastic elements are so securely joined to one another that when subjected to a pulling test, the plastic elements will themselves fail before the bond separates. For example, the ASTM D638-03 Standard Test Method for Tensile Properties of Plastics may be used to determine whether a successful weld has been formed. For retail containers intended to restrict pilferage, a welded heat seal is desirable.
Unfortunately, a weld between sheets of conventional PET material is difficult to form by heat sealing. Although PET is naturally a crystalline resin, it can be made into an amorphous, and hence transparent, material. Crystalline PET (CPET) is opaque. As the amorphous PET is heated above its crystallization temperature, it will return to a crystalline state, and will lose its transparency. Moreover, the material in this state will fail to form a suitable welded bond. The difficulty in forming a heat-seal weld between two PET sheets is this: In order to form a good weld, the plastic material must be heated above its glass transition temperature, but if the materials are heated too much they will become crystalline, and therefore incapable of properly bonding; yet the difference between the glass transition temperature and the crystallization temperature of conventional PET materials may be far too narrow an operating window for many conventional heat sealing processes.
PET may be modified by copolymerization to produce amorphous copolyesters such as PETG, available from EastmanChemical and SKchemicals. With amorphous copolyesters, the “operating window” of temperatures is expanded, and repeatable welding becomes more readily obtainable. PETG gives very good heat sealing performance. However, PETG is much more expensive than PET. Different resins may be combined into a single sheet of plastic material through the coextrusion process, in which two materials are simultaneously extruded to form a single sheet with distinct layers in intimate contact with one another. Although a lower-cost composite sheet might be formed with a thin layer of the more costly PETG coextruded with PET or RPET, the PETG is different enough from PET that it makes recycling difficult. For example, the glycol component of PETG can make recycling difficult. Hence where recycling of materials is desired PET and PETG should not be mixed.
What is needed is a sheet material for PET plastic container which can be economically fabricated and heat sealed to provide pilferage resistant seals, which is recyclable, and which is also readily used in connection with existing heat-sealing equipment designed for use with PVC materials.